Structural ceramics such as silicon nitride, silicon carbide, partially stabilized zirconia and partially stabilized alumina are being developed for high temperature applications in heat engines of various types (automotive, gas turbine), tools and dies, and other components because of their high strength at elevated temperatures, and errosion and corrosion resistance. However, because of the inherent brittleness (low fracture toughness) of ceramics, their application potential is not as widespread as it could be if their fracture toughness and ductility were enhanced.
The fracture strength of a ceramic is expressed by the relationship: EQU .sigma..sub.f .alpha.K.sub.c (c).sup.1/2
where K.sub.c is the fracture toughness (resistance to crack propogation) and c is the size of the critical crack that leads to failure. For a fixed flaw size distribution, therefore, strength is dependent on fracture toughness. Improvements in fracture toughness would undoubtedly enhance the use of such ceramics, especially where resistance to any thermal cycling is desired.
It has been proposed by F. F. Lange, "The Interaction of a Crack Front with a Second Phase Dispersion", Philosophical Magazine, 22 (1970) pp. 983-92 and further developed by A. G. Evans, "The Strength of Brittle Materials Containing Second Phase Dispersion", Philosophical Magazine, 26 (1972) pp. 1327-44, that the strength (hardness) and fracture toughness of ceramics can be increased by crack pinning by dispersed particles of a second phase. The concept is based on an observation of an increase in the length of a crack front when it interacts with two or more inhomogeneities. The theory further developed by Evans showed that significant improvement in fracture energy (toughness) can be achieved by a presence of closely spaced second phase particles.
Evidence of improvement of fracture toughness by a dispersion of voids throughout a solid material was provided by G. F. Hurley and F. W. Clinard, Jr. "Fracture Toughness and Hardness of Neutron-Irradiated A1.sub.2 O.sub.3, MgAl.sub.2 O.sub.4 and Y.sub.3 Al.sub.5 O.sub.12 ", DOE/ER-0048/1 (1981) pp. 51-57. They observed an increase in fracture toughness by a factor of 2 in a single crystal alumina which had been neutron irradiated. The irradiation produced an array of voids. They also found a good correlation between theoretically predicted fracture toughness and that observed, assuming the voids to be impenetrable obstacles. While neutron irradiation, with the resultant production of voids can increase the fracture toughness of ceramics, as a technique for producing ceramic with high toughness, such irradiation has several drawbacks. It is costly, it produces an anisotropic distribution of voids, and it renders the material radioactive.
In the present method, bubbles of a gas, such as helium, are homogeneously dispersed throughout a ceramic article to increase the fracture toughness of the ceramic, with the article formed by a powder metallurgical technique.